The invention relates to the field of disposable biomedical electrodes for establishing an electrical connection between the skin of the human anatomy and an electromedical apparatus such as a high impedance electromyograph, electrocardiograph, electrostimulator for pain relief and the like. An automated method of making disposable electrodes is also included within the scope of the invention.
Many types of disposable biomedical electrodes have been introduced in recent years. Typically they comprise (1) a metallic or electrically-conductive carbon terminal member having means for electrical connection to an electromedical apparatus such as an electrocardiograph; (2) an adhesive tape or pad for holding the terminal member in place on the skin; and (3) an electrically-conductive, conformable interfacing material such as an electrolyte gel or paste over the surface of the terminal member which contacts the skin to reduce skin impedance and improve electrical contact between the skin and the terminal member.
Disposable biomedical electrodes have been improved in the last few years by the introduction of electrically conductlve pressure-sensitive adhesives to replace conventional electrolyte gels and creams. Such gels are generally messy and unpleasant to use and, because of their high water content, require elaborate packaging to prevent "dry-out" problems. When an electrically-conductive pressure-sensitive adhesive is used as the interfacing material between the terminal member and the skin, a separate adhesive tape or pad to adhere the electrode to the skin may not be necessary. resulting in a smaller, lower-cost disposable electrode.
For ease of connectionto the lead wire of an electromedical device, the terminal member of many disposable biomedical electrodes is formed in the shape of the male portion of a snap fastener adapted to engage the female portion of the snap fastener attached to the lead wire of the electromedical device. Such a terminal member comprises a flat, circular portion and a knob-like projection extending perpendicularly therefrom. Such a one-piece terminal member is illustrated in U.S. Pat. No. 4,273,135. A one-piece terminal member of this type is difficult to securely anchor to the adhesive pad. If the terminal member is not securely anchored to the other components of the disposable electrode, it may pull out when force is applied to disconnect the terminal member from the female portion of the snap fastener.
To avoid this problem, a two-piece terminal member is typically used comprising a so-called "stud and eyelet" arrangement. The stud comprises a flat circular portion and a pin-like member extending perpendicularly therefrom. The stud is inserted into an eyelet comprising a hollow knob having a circular base. The adhesive tape or stiffening piece used to reinforce the adhesive tape is sandwiched between the stud and the eyelet. After the stud is inserted into the eyelet, the eyelet is crimped at its neck to ensure good electrical contact between the stud and eyelet and securely anchor the adhesive tape or stiffening piece. Alternatively, the stud can be friction fitted within the eyelet without crimping.
The two-piece terminal member as described above suffers from a number of disadvantages including potential corrosion problems if the stud and eyelet are made from dissimilar metals, electrical resistance between the stud and eyelet, and increased difficulty and cost of manufacturing.
Because of the disadvantages associated with the two-piece terminal member, particularly its high cost, a number of attempts have been made to solve the anchoring problems encountered with one-piece terminal members so that the latter might be more suitable for disposable bioelectrodes.
U.S. Pat. No. 3,841,312 describes a retaining ring which fits over the projecting portion of the terminal member to mechanically secure the adhesive tape to the flat base portion of the terminal member. U.S. Pat. No. 3,993,049 discloses a bioelectrode having a one-piece terminal member which is mechanically attached to the other layers of the electrode by a swaged stiffening member. U.S. Pat. No. 4,029,086 describes a disposable bioelectrode having a one-piece terminal member in which the projecting portion thereof contains a ridge over which a washer is snapped to secure the terminal member to the adhesive pad. U.S. Pat. No. 4,117,846 describes a similar method utilizing a pair of washers to secure the terminal member to the adhesive pad.
In general, the prior art methods of anchoring the one-piece terminal member to the other components of the electrodes have involved a friction-fit washer or retaining ring whict is mechanically biased against the stem of the knob-like projection of the terminal member. Although such methods of anchoring generally produce satisfactory anchoring of the terminal member to the electrode, they complicate the manufacture of the electrode and increase its cost.
The biomedical electrode of the present invention overcomes the disadvantages of the prior biomedical electrodes by securely anchoring the one-piece terminal member within the electrode without the necessity of washers or other retaining means that are mechanically biased against the projecting portion of the terminal member. Ths dramatically simplifies the method of manufacture of the electrode without compromising its mechanical integrity. A further aspect of the invention involves the use of a metal layer in conjunction with the anchoring means of the present invention to improve the electrical performance of the electrode and permit the use of a wide variety of ionically conductive mediums including conductive adhesives.